The results of numerous clinicopathological investigations with brain damaged patients have indicated that the nucleic of the diencephalon, particularly in the nucleus medialis dorsalis and the mammillary nuclei, are involved in the processing and storage of incoming sensory information. Within the past three years, we have developed and refined a non-stereotaxic surgical procedure for exposing and directly approaching the mammillary nuclei in non-human primates. We have used our micro-surgical technique to experimentally examine both global and specific changes in learning and memory capacity in non-human primates after the mammillary bodies have been removed. Our data suggests that although the mammillary bodies are not important for the retention of familiar or fixed information, they are important for the acquisition of new cognitive and spatial skills as well as remembering relative positions in space across long intervals of time. It is our intention to replicate and extend some of our findings on the functions of the mammillary nuclei and to compare the effects of lesions within this structure to lesions of the medial dorsal and medial pulvinar nuclei of the thalamus. Achieving our objectives should contribute significantly to the enhancement of our present understanding of the functions of the diencephalon in learning and memory in man as well as in non-human primates. Moreover, the results of such carefully controlled neurobehavioral experiments with laboratory primates should prove to be a useful clinical tool in the future assessment, treatment and rehabilitation of patients with memory disorders.